Electron beam forming apparatus



Dec. 8, 1959 T. D. SEGE 2,916,659

ELECTRON BEAM FORMING APPARATUS Filed March 28, 1956 INVENTCR ATTORNEY A :the amplifier.

United States Patent- This invention relates to electron beam forming apparatus for high power, high frequency electron tubes. More particularly, the invention concerns improvements in electron guns of the type employing electrostatic focusing'appara'tusintermediate the cathode and anode elements' of such guns.

-Electron guns employing electrostahc focusing have been described in a number of patents including US. Patents 2,268,196 and 2,564,743. Such focusing produces a concentrated stream of electrons by theagency of -a suitably-biased electrode member or members disposed intermediate a' cathode emitting surface and an accelerator anode,;through which member or members the electron"str eam passes by way of a coaxially-located aperture or apertures. i

* While electrostatic focusing has been widely adopted to general advantage in the design of electron guns, a disadvantage'has been noted in some cases which can preclude the use of electrostatic focusing whereit would otherwise be highly desirable. This disadvantage is that the space defined in substantial measure by the-electrostatic focusing apparatus can operate as a cavity resonator 'thatwill sustain oscillations initiated bythe passage of i the electron stream therethrough. Moreover, these oscillations j'arelikely to occur in voltagemodesthat interact -'with the negative resistance region in the beam voltage vs'. beam current characteristic, whereby to cause density orvelocity modulation of theelectron stream before it Y 'fil'eaves the cathode anode region of v the electron gun.

While the power in such oscillations usually is relatively -small, the consequent beam modulation, when, amplified through the cavities of a klystron or by a traveling wave tube,v may become comparable'to the poweroutput of One way of preventing offending oscillations within a pa'r ticularelectron gun structure would apparently be to --modify the; focusing electrode configuration so as to change the volume of the'cavity it aids to determine. The volume change may then push the undesired voltage mode out of the resonant region of the cavity. However, in so doing, it mayithereby introduce another voltage mode of different order into the same region. Therefore, changing. the volume of the cavity is clearly :an unsatisfactory approach toward solving the problem.

Another approach that would apparently prevent the ofiending electron gun oscillations would be the introduction: oflossy material, such as lossyceramic, into the electron gun structure. However, the amount and location of such material would be so critical and the mount- :ing difficulties so great that this approach to solving the problem must be abandoned as being highly impractical. By the present invention, a solution is presented which 'not only prevents the ofiending oscillations, but also teriously with the electron beam if the oscillationsare and the TM family of modes in particular.

Patented Dec. 8, 1959 excited in one of the cavity modes that is characterized by electric fields in general alignment with the electron beam. This would apply to the TM modes in general, Due to asymmetries in the beam, interaction between the beam and the TM family of cavity modes may also be possible. All these modes are characterized by axial current flow in the cavity wall. The interacting mode is, accordingly, suppressed by interrupting this current fiow with appropriately located slots or elongated apertures in the cavity structure, whereby to distort the fields tending to be produced by the mode and thus, in most cases, eliminate the likelihood of exciting the mode.

If, however, a particular case should arise where, due in part to the capacitive coupling across the slots, enough current can still flow to permit the mode to come into being, the problem is met by introducing a highly lossy material into the slots, such as by impregnating the slots with lossy porous ceramic. Thus, if the resultant Q of the electron gun cavity is not lowered enough by the slots themselves to eliminate the offending oscillations, then this is certainly accomplished by the slots together with lossy material therein.

The principal object of the present invention is to provide a novel arrangement for suppressing oscillations that tend'to occur and be sustained in electron beam forming apparatus and which, if permitted to occur, would interact deleteriously with the beam formed by such apparatus.

Another object is the provision of an electron beam forming apparatus including an electrostatic focusing electrode wherein the space defined in substantial measure by said electrode is prevented by novel means from forming part of a cavity resonator that would otherwise tend to sustain oscillations deleteriously interacting with the electron beam.

Another object is to provide, in an electron beam forming apparatus, a focusing electrode having a plurality of elongated apertures or slots which are positioned to suppress beam-modulating current flows in the structure of the electrode.

With the foregoing and other objects in view, the present invention includes the novel elements and combinations and arrangements thereof described below and illustrated in the accompanying drawings, in which:

Fig. 1 is a side view, partly in section, of an electron gun embodying the present invention; and

. Figs. 2 and 3 are like perspective views, partly in section, of a focusing electrode provided with slots according to the present invention, respectively with and without lossy material in said slots.

The electron gun depicted in Fig. 1 is assumed to be installed in a high power, high frequency electron tube such as, for example, the type including klystrons and travelingwave tubes. The cathode portion of the electron gun comprises a spherically concave electron emitting element 5 secured by supports 6, 7 to a surrounding shell 8 rigidly fastened to a flanged portion 9 of the stem 10 of the gun. Aheater element 11 for the concave emitter 5 may be energized by connecting a suitable potential source across the pins 12, 13 which extend outwardly from the base of the neck 14 of the tube and inwardly of stem 10 through a supporting web 15 to connect to the respective end terminals of the heater element. A spaced pair 16 of heat shields are mounted directly behind the heater element so as to enhance the transfer of heat to emitter 5.

An anode 17 having a frusto-conical tapered aperture 18 coaxial with the circular rim of emitter 5 and the longitudinal axis of the electron gun is axially spaced from the emitter and arranged so that it may be employed for accelerating the emitted stream of electrons in a path toward the main body of the electron tube. The-Wide month of aperture 18 faces toward emitter 5 and is of lesser diameter than the emitters rim.

In the space intermediate emitter 5 and anode 17 is an electrostatic focusing electrode 19 of generally hollow cylindrical configuration supported by and partially fitted within shell 8 so as to be coaxial with aperture 18 and the rim of the emitter. By this arrangement, the wall of electrode 19 describes a figure of revolution about the electron beam. The axial length of focusing electrode 19 is slightly less than the axial distance between emitter 5 and the wide mouth of the anode aperture 13. The focusing electrodes rim proximate the rim of emitter 5 is turned inwardly to form a rim portion 29 depending toward the axis of the electron gun and having an inner diameter substantially equal to the emitters rim diameter. At the other end of the focusing electrode, the rim thereof proximate the wide mouth of anode aperture 18 is likewise turned inwardly to form a rim portion 21 depending toward the gun axis. But rim portion 21 has an inner diameter slightly less than that of rim portion 20, yet somewhat greater than that of the adjacent anode aperture;

Preferably, the wall of focusing electrode 19 is of increased thickness in the end region thereof including rim portion 21, so as to increase the heat-withstanding ability of electrode 19 in this region. Preferably, also, the possibility of voltage breakdown between rim portion 21 and anode 17 is reduced to a minimum by rounding off the edges that would otherwise be formed by the axially-depending sides of rim portion 21.

In accordance with well-known principles, rim portions 20, 21 and the cylindrical wall joining them are adapted to cooperate in electrostatically countering the space charge effect of the electron stream that tends to disperse the electronsfrom a desired highly dense converging path .or beam of circular cross-section having a minimum convergence within anode aperture 13. From the view of lClfODOptiCS, focussing electrode 19 is an electrostatic converging lens in the path of the beam.

Generally in low power electron guns, the space outlined by the emitter, focussing electrode, and anode is quite lossy and, therefore, relatively unlikely to function as a resonant cavity. Moreover, the odds in favor of a resonance being obtained coinciding with the operating band of the electron tube are relatively low. In other words, it is in the case of high power, high density type electron guns, and especially those operating over a wide band that the space defined in substantial measure by the focussing electrode is most likely to operate as a cavity resonator having a resonance in the frequency range of the tube. Moreover, this resonance may occur in several different modes depending upon the manner in which the cavity is excited.

Assuming in Fig. 1 that emitter 5, focussing electrode 19, and anode l7 outline a resonant cavity, it is evident from the generally cylindrical configuration of the cavity longitudinally of the beam that if the cavity is excited by the traversal of the beam, such excitation will be in a TM mode. Thus, there will be mode-supporting oscillatory currents or electron flows in the cylindrical wall of focusing electrode 19 moving longitudinally of said Wall and interacting with the beam to velocity modulate the same.

The interaction is particularly harmful if the oscillations fall within the operating range of the tube served by the electron gun. And, further, if the oscillations do so fall, they may occur in more than one order of the TM mode. Hence, by suppressing the TM mode orders in which the objectionable oscillations tend to occur, such oscillations are prevented from coming into being.

Accordingly, the focusing electrode 19 (see Fig. 2) is provided with a plurality of elon ated apertures or slots 22, in its periphery, which :slots are circularly aligned in a planesubstantially normal to the electrodes longitudinal. axis. The axially normal plane .of slots 22 is preferably located so that the slots are positioned in a .regionof the cylindrical Wall where maximum objectionable oscillatory current tends to flow. This region may be deduced through analysis of the resonant cavity to determine the TM mode orders that will support resonant oscillations in the frequency range of the tube served by the electron g The number of slots 22 and their length are notcritical factors. It is preferable, however, to limit the spacing betweent adjacent slot ends to the minimum amount necessary to preserve structural rigidity in electrode 719, whereby the disruptive elfect of the slots on the objectionable oscillatory currents that tend to flow in the cavity wall may be attained to the, highest degree.

In a case where the minimum spacing that may be practicably provided between adjacent slot ends, together with the capacity coupling across the slots themselves, is sufficient to permit enough objectionable oscillatory current to flow, the slots may be filled with a highly lossy material 23 (see Fig. 3). A suitable material in this regard would be a porous ceramic, carbon impregnated, for example, which may be readily placedin'to the slots.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in theyaccompanying drawings shall be interpreted as illustrative and not in a limiting sense. r

What is claimed is:

1. Electron beam forming apparatus including a focussing electrode having a wall describing a figure of revolution about the electron beam and defining insubstantial measure the boundaries of a resonant cavity tending to be excited by said beam, said wall'of said electrode having a plurality of elongated apertures whose length dimensions lie across the direction of current flow along the inner surface of said well forundesired TM modes in the resonant cavity that modulatersaid electron beam, whereby said undesired modes are substantially suppressed.

2. An electron gun for a'high power, high frequency electron tube having a predetermined range of operating frequencies, said gun including a focussing electrode having a longitudinally-extending wall outlinin'g insubstantial measure the boundaries of a cavity resonator of resonant frequency falling within said predetermined frequency range, said Wall including a plurality of elongated slots aligned in a plane transverse'towsaid wall and positioned so as to substantially preventoscillatory cur rents of said resonant frequency from occurring in said wall.

3. In an electron beam forming apparatusof the type including a cathode and anode, a focussing electrode disposed intermediately of. said cathode and anode, said electrode having a Wall surrounding the "electron-beam, said wall being provided with a plurality of :apertures arranged to suppress oscillatory currents of motion longitudinally of said beam that tend in the absence of such suppression to be excited in said wall -by:the passage of said beam from said cathode to said anode.

4. Electron gun apparatus for producing and directing an electron stream along a predetermined path, comprising a cathode electron emitter positioned at one end of said path, an anode positioned along saidpath in spaced relationship with said cathode, electrostatic focussing electrode means positioned alongzsaid path between said cathode and anode, said cathode, focussing electrode means and anode forming a boundary of a microwave cavity having undesired resonance properties :in an electromagnetic wave energy mode which interacts with said'electron stream, and means coupled to said cavity for inhibiting microwave'cavity oscillations in said mode to thereby'fmim'mize modulation "of .said electron rstream.

5. Electron gun apparatus as set forth in claim 4, wherein said focussing electrode includes a tubular wall extending along the path of said electron stream, and a plurality of apertures in said tubular wall in the path of electric current flow for TM mode electromagnetic energy to provide a high impedance to said current flow.

References Cited in the file of this patent UNITED STATES PATENTS 2,303,166 Laico Nov. 24, 1942 6 McRae June 24, 1947 Kinzer Mar. 14,1950 Brehm et a1 Oct. 31, 1950 Brehm Apr. 5, 1952 Hull Mar. 17, 1953 Broadway et al. July 28, 1953 Brewer Oct. 29, 1957 Poulter Feb. 4, 1958 FOREIGN PATENTS Great Britain Nov. 8, 1950 I 

